1. Field of the Invention
The present invention relates, in general to a process of removing radioactive or heavy metal contamination. In particular, the present invention relates to a process for decontaminating solid surfaces; to a chemical cleaning of steam generator sludge; and to a process for removing radioactive or heavy metal contamination from solutions.
2. Background Information
Many industrial processes result in the generation of undesirable solids or liquid waste streams containing hazardous metal species in forms such as solid oxides and dissolved ions. In particular, this pertains to the nuclear industry and to facilities which make use of radioisotopes, where the generation of wastes and of other undesirable materials, such as corrosion products accumulated on or around metal surfaces, requires continuous efforts to ensure that radioactive species present in such wastes or undesirable materials will not be released to the environment at levels capable of producing significant health risk or environmental damage. One common example of the need for effective control of radioactive species is the decontamination of surfaces of equipment in facilities using radio-isotopes. Another one is cleaning steam generators in nuclear power stations, where the accumulation of sludge is the cause of serious operational problems. Nuclear power plants and other facilities also generate liquid waste streams that require removal of radioactive contaminants.
Present techniques of managing radioactive contamination involve extensive use of complexing and chelating agents. The problem of decontaminating metal surfaces contaminated with radioactive species has been addressed using a relatively small number of chemical decontaminating agents. These can be divided, in general, into several major categories, which include pH control agents (acids, bases or buffers), oxidizing agents such as hydrogen oxide or potassium permanganate and reducing agents such as hydrazine, complexing or chelating agents, such as oxalate, citrate, gluconate, picolinate, and ethylenediaminetetraacetic acid (EDTA), and surfactants. Certain additives can have more than one role. For instance, ammonia and its salts can function both to control the pH and to act as complexing agents for certain metals such as copper. In addition, secondary constituents of decontaminating solutions include emulsifiers, corrosion inhibitors, etc. [J. A. Ayres, Decontamination of Nuclear Reactors and Equipment, The Ronald Press Company, New York, N.Y., 1970.] A typical decontamination operation may consist of several mechanical and chemical treatment steps.
The main requirements which a decontaminating agent should meet have been recently outlined. [J. Severa and J. Bar, Handbook of Radioactive Contamination and Decontamination, Studies in Environmental Science 47, Elsevier, Amsterdam, 1991.] In addition to being effective in removing contaminants from the metal surfaces of interest, a decontaminating agent should not be excessively corrosive and should be easy to prepare and to apply. The extent of surface corrosion caused by these agents should be controllable. No less important are considerations related to the environmental impact of the decontaminating agent itself. The presence of the decontaminating agent should not create excessive difficulties in treating the resulting waste stream, nor should this agent constitute a pollutant or promote pollution by other agents.
Most decontaminating agents do not fully meet all of these criteria. A major example of the resulting complications is provided by the widespread use of EDTA. EDTA is a highly effective chelating agent which is relatively easy to prepare, given proper control of the pH and temperature during its dissolution in water, and it does not pose serious health hazards.
Bulat (U.S. Pat. No. 3,047,434) describes a solution for radioactive decontamination comprising a mixture of water, EDTA, sodium sulfate, sodium hydroxide, manganese dioxide and carbon black, and a decontamination method which comprises immersing a radioactively contaminated article in such a solution, and subjecting the combination to ultrasonic vibration forces sufficient to produce cavitation. In addition to manganese dioxide and carbon black, other solids mentioned as being of possible use as ingredients of decontamination solutions are silicates such as colloidal clay, talc, fuller's earth, chalk, sulfides of arsenic and antimony, diatomaceous earth, and metallic oxides such as alumina, magnesia, iron oxide and titanium dioxide.
However, the presence of EDTA or of other stable complexing or chelating agents, in a waste stream contaminated with radioactive species poses major problems when attempts are made to reduce the volume of the waste stream by removing these contaminants with ion exchange or sorption beds. The presence of EDTA hinders the immobilization of the liquid waste in cement or epoxy media and limits the amount of volume reduction which can be achieved in this operation. Furthermore, the presence of EDTA at a disposal site dramatically enhances the dissolution of actinides in groundwater and their transport away from the site. [J. M. Cleveland, C&EN, Apr. 13, 1981, p.63.] Both federal and state regulations restrict the concentrations of chelating agents in solidified low-level waste forms, thus limiting achievable volume reduction factors with respect to spent decontamination media. It would be highly desirable to destroy the EDTA in the waste stream before attempting solidification or removal of radionuclides. However, EDTA has very high thermal and hydrolytic stability, and its destruction requires the use of very severe conditions, such as oxidation with hot concentrated hydrogen peroxide or incineration.
Another area of nuclear technology which involves the use of complexing and chelating agents is the removal of sludge from steam generators of nuclear power stations. Conventional techniques of sludge removal from steam generators in the nuclear industry are based on mechanical means (water jet lancing) alone. Mechanical cleaning is, in general, only partially effective within the time available for steam generator cleaning during outages.
Various methods of chemical cleaning have been developed and tested both in the USA and abroad with a view to enhancing the effectiveness of sludge removal. The EPRI/SGOG process, which involves treatment with an EDTA-based iron solvent followed by an ethylenediamine(EDA)-based copper solvent, [J. M. Jevec and W. S. Leedy, Chemical Cleaning Solvent and Process Testing, EPRI NP-2976, April 1983; D. Schneidmiller and D. Stiteler, Steam Generator Chemical Cleaning Process Development, EPRI NP-3009, April 1983] is the only process currently approved for use in USA power stations. The EPRI/SGOG process has already been used in full scale steam generator cleaning at several power stations. However, the use of the EPRI/SGOG process generates large volumes of radioactively contaminated concentrated EDTA solutions containing high levels of Fe, Cu and other metals. Disposal of such spent solutions is complicated and expensive because EDTA is highly resistant to thermal decomposition and oxidation, poses major difficulties during waste solidification, and is subject to severe regulatory restrictions on its concentration in solidified wastes when such wastes are shipped to radioactive waste burial sites.
The use of complexing and chelating agents on solids supports has been described. For instance, U.S. Pat. No. 4,222,892, by Motojima et al., describes a process for adsorbing oxine (8-hydroxyquinoline) on activated charcoal and using the complex to remove radionuclides from the cooling water and leakage-water of nuclear reactors. A mention is also made of a method of removing heavy metals such as mercury, copper, chromium, etc., from a water solution containing the heavy metals using an activated charcoal or silica gel on which oxine is adsorbed and carried. However, as in the cases of spent decontamination media and the streams generated upon chemical cleaning of contaminated steam generator sludge, the presence of complexing or chelating agents together with radioactive contaminants in the secondary waste generated in the process, i.e., the spent oxine-impregnated charcoal, greatly complicates the disposal of this secondary waste in a burial site.
This invention relates to the use of non-persistent complexing agents, in particular hydroxamic acids, as substitutes for conventional complexing or chelating agents in cleaning operations involving hazardous metal species, in particular radioactive metal species, with a view to minimizing the amount of secondary wastes resulting from said cleaning operations.